This invention relates to a method for manufacturing a cylindrical frame for an electric motor, and more particularly to a method for manufacturing an electric motor frame having a metallic member such as a cooling fin welded to the outer surface of the main body of the frame.
In order to dissipate heat generated in an electric motor during its operation, a plurality of cooling fins are provided on the outer cylindrical surface of the motor housing or the main body of the motor frame. A method has already been proposed of manufacturing a motor frame including a main body and a metallic member welded to the outer surface of the main body. According to this proposed method, a blank metallic sheet is rounded into the shape of a cylinder by a rolling machine such as a three-roller bending machine, and seam welded at the joint. This tube is then subjected to sizing by a plurality of expansion pawls to radially expand and plastically form the tube so that the inner circumferential surface of the tube defines a true circle. The tube is then provided at its outer peripheral surface with a plurality of cooling fins attached by projection welding, for example. A stator core having a plurality of coils within slots is inserted into the frame thus obtained to complete a motor stator.
With the motor frame thus manufactured, in order to effectively dissipate the heat generated during the operation of the motor, the area of contact area or the welded area between the cooling fins and the outer surface of the frame main body must be as large as possible. However, if the welded area between the fins and the frame main body is increased, the welding heat added to the main body is large, resulting in the thermal deformation or distortion of the frame main body due. The thermal distortion effects radius of curvature of the inner circumference of the tubular frame main body and causes the circumferential expansion of the frame main body itself at the welded portions. When the stator core is inserted into the motor frame which is distorted as described above, a local air gap is formed between the stator core and the inner surface of the frame. Therefore, the heat conduction between the inserted core and the frame is degraded due to the air gap, decreasing the heat dissipation through the cooling fins disposed on the outer circumference of the frame main body.